U.S. patent number 7,524,561 [Application Number 12/127,877] was granted by the patent office on 2009-04-28 for coated water-swellable material.
This patent grant is currently assigned to The Procter & Gamble Company. Invention is credited to Bruno Johannes Ehrnsperger, Stephen Allen Goldman, Axel Meyer, Mattias Schmidt, Edward Joseph Urankar.
United States Patent |
7,524,561 |
Schmidt , et al. |
April 28, 2009 |
**Please see images for:
( Certificate of Correction ) ** |
Coated water-swellable material
Abstract
This invention is directed to coated water-swellable materials,
typically solid, particulate, water-swellable materials, i.e.
materials that comprise hydrogel-forming polymers, whereof at least
a part is coated with a coating, which substantially does not break
when the polymers swell, as set out in the method herein. Said
coating is present at a level of at least 1% by weight of the
water-swellable material. The coating comprises preferably an
elastomeric polymeric material. The invention also relates
products, e.g., disposable absorbent articles, comprising such
coated water-swellable material.
Inventors: |
Schmidt; Mattias (Idstein,
DE), Meyer; Axel (Frankfurt am Main, DE),
Ehrnsperger; Bruno Johannes (Mason, OH), Goldman; Stephen
Allen (Montgomery, OH), Urankar; Edward Joseph (Mason,
OH) |
Assignee: |
The Procter & Gamble
Company (Cincinnati, OH)
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Family
ID: |
34135181 |
Appl.
No.: |
12/127,877 |
Filed: |
May 28, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080226898 A1 |
Sep 18, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11890721 |
Aug 7, 2007 |
7402339 |
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10912002 |
Aug 5, 2004 |
7270881 |
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60492932 |
Aug 6, 2003 |
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Current U.S.
Class: |
428/407;
428/423.1; 428/327 |
Current CPC
Class: |
C09D
133/14 (20130101); C08J 7/0427 (20200101); C08J
7/056 (20200101); C08J 3/12 (20130101); A61L
15/60 (20130101); C08J 3/126 (20130101); C08J
2421/00 (20130101); Y10T 428/31551 (20150401); C08J
2300/14 (20130101); Y10T 428/2998 (20150115); Y10T
428/269 (20150115); Y10T 428/254 (20150115) |
Current International
Class: |
B32B
5/16 (20060101) |
Field of
Search: |
;428/327,423.1,407 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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509708 |
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Oct 1992 |
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EP |
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752892 |
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Jan 1997 |
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EP |
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799258 |
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Mar 2001 |
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EP |
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56-159232 |
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Dec 1981 |
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JP |
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57-168921 |
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Oct 1982 |
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JP |
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02-242858 |
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Sep 1990 |
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JP |
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09-031203 |
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Feb 1997 |
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JP |
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2000-198858 |
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Jul 2000 |
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JP |
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WO 90/08789 |
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Aug 1990 |
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WO |
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WO 92/16565 |
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Oct 1992 |
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WO |
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WO 93/05080 |
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Mar 1993 |
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WO |
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Primary Examiner: Le; H. (Holly) T
Attorney, Agent or Firm: Powell; John G.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No.
11/890,721, filed Aug. 7, 2007, now U.S. Pat. No. 7,402,339, which
is a continuation of U.S. application Ser. No. 10/912,002, filed
Aug. 5, 2004, now U.S. Pat. No. 7,270,881, which claims the benefit
of U.S. Provisional Application No. 60/492,932, filed Aug. 6, 2003,
all of which are incorporated herein by reference.
Claims
What is claimed is:
1. A water-swellable material that comprises hydrogel forming
polymers, said water swellable material having a CCRC of at least
10 g/g, said hydrogel forming polymers being coated by a first and
second coating wherein at least one of said first and second
coating is formed from a coating agent comprising an elastomeric
polymeric material, said coating agent being present at a level of
at least 1% by weight of the water-swellable material wherein, for
at least a part of the coated hydrogel forming polymers, said
coating is non-breaking when the water-swellable material is
swollen to equilibrium in 0.9% saline solution and wherein said
part of the coated hydrogel forming polymers that has a
non-breaking coating of an elastomeric polymeric material is at
least 60% by weight of the material having a coating with an
elastomeric, polymeric material.
2. A water-swellable material according to claim 1, wherein at
least one of the first and second coating has an average thickness
of at least 5 .mu.m.
3. A water-swellable material according to claim 1, wherein at
least one of the first and second coating has an average thickness
of at least 10 .mu.m.
4. The water-swellable material according to claim 1, wherein at
least one of the first and second coating covers at least about 80%
of the surface of the hydrogel forming polymer.
5. The water-swellable material according to claim 1, wherein at
least one of the first and second coating is completely
continuous.
6. The water-swellable material according to claim 1, wherein at
least one of the first and second coating is completely
connected.
7. The water-swellable material according to claim 1, wherein the
first coating comprises the coating agent.
8. The water-swellable material according to claim 1, wherein the
first and second coating each comprise the coating agent.
9. The water-swellable material according to claim 1, wherein the
first coating comprises the coating agent and the second coating
comprises an organic or inorganic powder.
10. The water-swellable material according to claim 9, wherein the
organic or inorganic powder comprises at least one of a salt, a
silicate, and clay.
11. The water-swellable material according to claim 1, wherein at
least one of the first and second coating completely circumscribes
the hydrogel forming polymer.
12. The water-swellable material according to claim 1, wherein the
hydrogel forming polymer has a mass median particle size of from
about 10 .mu.m to about 1 mm.
Description
FIELD OF THE INVENTION
This invention is directed to coated water-swellable materials,
typically solid, particulate, water-swellable materials, i.e.,
materials that comprise hydrogel-forming polymers, whereof at least
a part is coated with a coating agent with a polymeric elastomeric
material, which substantially does not rupture when the polymers
swell, as set out in the method herein.
The coating agent comprises preferably an elastomeric polymeric
material. The invention also relates products, e.g., disposable
absorbent articles, comprising such coated water-swellable
material.
BACKGROUND OF THE INVENTION
An important component of disposable absorbent articles such as
diapers is an absorbent core structure comprising water-swellable
polymers, typically hydrogel-forming water-swellable polymers, also
referred to as absorbent gelling material, AGM, or super-absorbent
polymers, or SAP's. This polymer material ensures that large
amounts of bodily fluids, e.g., urine, can be absorbed by the
article during its use and locked away, thus providing low rewet
and good skin dryness.
Especially useful water-swellable polymer material or SAP's are
often made by initially polymerizing unsaturated carboxylic acids
or derivatives thereof, such as acrylic acid, alkali metal (e.g.,
sodium and/or potassium) or ammonium salts of acrylic acid, alkyl
acrylates, and the like in the presence of relatively small amounts
of di- or poly-functional monomers such as
N,N'-methylenebisacrylamide, trimethylolpropane triacrylate,
ethylene glycol di(meth)acrylate, or triallylamine. The di- or
poly-functional monomer materials serve to lightly cross-link the
polymer chains thereby rendering them water-insoluble, yet
water-swellable. These lightly crosslinked absorbent polymers
contain a multiplicity of carboxyl groups attached to the polymer
backbone. It is generally believed, that these carboxyl groups
generate a driving force for the absorption of body fluids by the
crosslinked polymer network.
In addition, the polymer particles are often treated as to form a
surface cross-linking `coating` on the outer surface in order to
improve their properties in particular for application in baby
diapers.
Water-swellable (hydrogel-forming) polymers useful as absorbents in
absorbent members and articles such as disposable diapers need to
have adequately high sorption capacity, as well as adequately high
gel strength. Sorption capacity needs to be sufficiently high to
enable the absorbent polymer to absorb significant amounts of the
aqueous body fluids encountered during use of the absorbent
article. Together with other properties of the gel, gel strength
relates to the tendency of the swollen polymer particles to deform
under an applied stress, and the gel strength needs to be high
enough so that the particles do not deform and fill the capillary
void spaces in the absorbent member or article to an unacceptable
degree, so-called gel blocking. This gel-blocking inhibits the rate
of fluid uptake or the fluid distribution, i.e. once gel-blocking
occurs, it can substantially impede the distribution of fluids to
relatively dry zones or regions in the absorbent article and
leakage from the absorbent article can take place well before the
water-swellable polymer particles are fully saturated or before the
fluid can diffuse or wick past the "blocking" particles into the
rest of the absorbent article. Thus, it is important that the
water-swellable polymers (when incorporated in an absorbent
structure or article) maintain a high wet-porosity and have a high
resistance against deformation thus yielding high permeability for
fluid transport through the swollen gel bed.
Absorbent polymers with relatively high permeability can be made by
increasing the level of internal crosslinking or surface
crosslinking, which increases the resistance of the swollen gel
against deformation by an external pressure such as the pressure
caused by the wearer, but this typically also reduces the absorbent
capacity of the gel undesirably.
The inventors found that often the surface crosslinked
water-swellable polymer particles are constrained by the
surface-crosslinking `shell` and cannot absorb and swell
sufficiently, and/or that the shell is not strong enough to
withstand the stresses of swelling or the stresses associated with
performance under load.
The inventors have found that the coatings or shells of the
water-swellable polymers, as used in the art, including surface
cross-linking `coatings`, break when the polymer swells
significantly or that the `coatings` break after having been in a
swollen state for a period of time. They also found that, as a
result thereof, the coated and/or surface-crosslinked
water-swellable polymers or super-absorbent material known in the
art de-form significantly in use thus leading to relatively low
porosity and permeability of the gel bed in the wet state. They
found that this could be detrimental to the optimum absorbency,
liquid distribution or storage performance of such polymer
materials.
Thus, the inventors found that what is required are water-swellable
materials comprising coated water swellable polymers that have a
coating that can exert a force in the wet state and that does
substantially not rupture when the polymers swell in body liquid
under typical in-use conditions. In the context of this invention,
the inventors have found that as a good representative for body
liquids such as urine, a 0.9% sodium chloride by weight in water
solution, further called "0.9% saline" can be used. Therefore the
inventors have found that it is required to have coated water
swellable materials, or coated hydrogel forming polymers, where the
coating does substantially not rupture when the materials swell in
0.9% saline.
The inventors have now developed a new water-swellable material
comprising hydrogel forming polymers, of which at least a part is
coated with a coating agent, which is elastomeric, so that when the
internal core of material swells (and forms a hydrogel), the
coating can extend and remains substantially intact, i.e., without
breaking.
The inventors further found improved or preferred processes of
applying and subsequently treating the coatings, as to obtain
preferred material of the present invention, with further improved
properties.
SUMMARY OF THE INVENTION
The present invention relates to water-swellable material that
comprises hydrogel-forming polymers coated by a coating, formed
from a coating agent comprising an elastomeric polymeric material,
whereby said coating is present at a level of at least 1% by weight
of the water-swellable material, and whereby the water swellable
material has a CCRC of at least 10 g/g (or even at least 20 g/g or
even at least 30 g/g) and whereby, for at least a part of the
coated hydrogel forming polymers, said coating is non-breaking,
when the water-swellable material is swollen to equilibrium in 0.9%
saline solution by the method defined herein.
Said part of the coated hydrogel forming polymers that has a
non-breaking coating of an elastomeric polymeric material is at
least 60% by weight, or even at least 80% or even at least 90% or
even at least 95% or even 100% by weight of the material, having a
coating with an elastomeric, polymeric material.
The coating agent is applied such that the resulting coating layer
is preferably thin; preferably the coating layer has an average
caliper (thickness) between 1 micron to 200 microns (.mu.m), more
preferably from 1 micron to 100 microns or even to 50 microns or
even to 20 microns, or even more preferably from 2 to 15
microns.
The water-swellable material and the hydrogel forming polymers are
preferably solid, preferably particulate.
The coating agent comprises preferably natural or synthetic
elastomeric polymeric materials, preferably elastomeric polymeric
materials selected from the group of natural rubber, synthetic
rubber and thermoplastic elastomers that are elastic at 35.degree.
C.
The inventions also relates to absorbent structures, suitable for
(disposable) absorbent articles, comprising the water-swellable
material of the invention, and to such absorbent articles, such as
diapers.
The intention also relates to a process for making the
water-swellable material of the invention by the steps: a)
obtaining hydrogel forming polymers; and b) simultaneously with or
subsequently to step a), applying a coating agent, comprising an
elastomeric material, to at least part of said hydrogel forming
polymers to obtain coated hydrogel forming polymers; and preferably
c) prior to, simultaneous with or subsequent to step b), obtaining
said hydrogel forming polymers or coated hydrogel forming polymers
in solid, preferably particulate, form.
The process preferably comprises a curing step, to cure the
coatings, e.g., preferably subsequently or simultaneously with step
c), the coated polymers are subjected to a temperature of at least
80.degree. Celsius, preferably at least 100.degree. Celsius, more
preferably at least about 140.degree. Celsius.
The invention also relates to the use of the process above to
increase the porosity of the hydrogel-forming polymers in the wet
state.
DETAILED DESCRIPTION OF THE INVENTION
Water-Swellable Material, Hydrogel Forming Polymers and Resulting
Coatings
The water-swellable material of the invention is such that it
swells in water by absorbing the water, thereby forming a hydrogel.
It may also absorb other liquids and swell. Thus, when used herein,
`water-swellable` means that the material swells at least in water,
but typically also in other liquids or solutions, preferably in
water based liquids such as 0.9% saline.
The water-swellable material of the invention comprises at least
60% by weight (of the material) of hydrogel forming polymers that
are coated with an elastomeric, polymeric coating agent that forms
a non-breaking coating.
This can be determined by observation of the coating of the
material of the invention, by any known method for observing the
surface structure or coating of solid materials, such as with the
methods described below.
Hereby, a certain weight amount of the water-swellable material is
stained and swollen to its equilibrium (e.g., the hydrogel forming
polymer particles and coated hydrogel polymer particles) and then,
by use of equipment described herein, one can visually distinguish
the particles with a non-breaking coating and the material without
non-breaking coating, and separate these into two fraction and then
determine the weight of each fraction and determine the weight
percentage of hydrogel forming polymer particle which are coated
with a non-breaking coating, as described herein, e.g., by mere
counting and calculating.
In practice, when using the method described herein, the coating of
a swollen coated hydrogel-forming polymer particle is observed as,
and considered as, non-breaking when either: at least 80% of the
surface of a (swollen) hydrogel forming polymer particles is
covered by the elastomeric, polymeric coating; and/or when the
coating is observed as completely continuous and/or completely
connecting and/or completely circumscribing the core of the
particle; and/or when no break lines or cracks, which divide the
coating into two separate parts, are observed.
Typically, the coating layer or shell of is connected; more
preferably, the coating shell is connected and completely
circumscribing the hydrogel forming polymer (s), both before and
after swelling in 0.9% saline, by the method described herein.
For the purpose of a preferred embodiment of this invention, the
coating is considered connected when for each two points P1 and P2
that are in the coating of a hydrogel forming polymer particle,
there is at least one continuous line that connects these points P1
and P2 and that completely lies within the coated shell.
For the purpose of a preferred embodiment of this invention, the
coating is considered completely circumscribing the hydrogel
forming polymer(s) when for each point P3 positioned in the
hydrogel forming polymer (and thus not on or in the coating shell
or layer) and for each point P4 outside the coated hydrogel forming
polymer particle or water-swellable material, all continuous bands
having a circular cross-section that connect P3 and P4 and that
have a diameter of 500 .mu.m, or preferably even only 100 .mu.m,
will intersect the coated layer/shell. (A band is defined as a line
with a circular cross-section.)
It should be understood for the purpose of the invention that not
all hydrogel forming polymers in the water-swellable material has
to be coated with the coating agent herein.
However, at least 60% of the hydrogel forming polymer particles,
coated with the coating agent herein, have a non-breaking coating,
when the water-swellable material (as a whole) is swollen to
equilibrium in a 0.9% saline solution. Preferably this percentage
is even higher, e.g., at least 70%, or even at least 80% or even at
least 90% or even at least 95%.
For the purpose of the invention, the hydrogel forming polymers of
which at least a part is coated is herein referred to as `coated
hydrogel forming polymers (particles)`, unless specifically defined
differently.
The coating agent is preferably present at a level of 1% to 50% by
weight of the water-swellable material, more preferably from 1% to
30% by weight or even from 1% to 20% by weight or even from 2% to
15% by weight.
In particular in this embodiment, the coating materials and the
resulting coatings are preferably highly water permeable such as to
allow a fast penetration/absorption of liquid into the water
swellable material (into the core).
In another preferred embodiment of the invention, the coating shell
is porous and in the form of a network comprising pores for
penetration of water, such as for example in the form of a fibrous
network, that is connected and circumscribing the particle as
defined above.
The coating agent is applied such that the resulting coating layer
is preferably thin; preferably the coating layer has an average
caliper (thickness) between 1 micron to 200 microns (.mu.m), more
preferably from 1 micron to 100 microns or even to 50 microns or
even to 20 microns, or in certain embodiments, even more preferably
from 2 to 15 microns.
The coating is preferably uniform in caliper and/or shape.
Preferably, the average caliper is such that the ratio of the
smallest to the largest caliper is between 1:5 to 1:1, preferably
1:2 to 1:1. Thus, preferred is that the average caliper or
thickness is in about the same range as cited above.
The water-swellable material of the invention may also comprise
other components, such as fillers, flowing aids, process aids,
anti-caking agents, odor control agents, colouring agents, coatings
to impart wet stickiness, hydrophilic surface coatings, etc.
However, the hydrogel forming polymer particles of which at least a
part is coated, are preferably present in the water-swellable
material at a level of at least 60% by weight (of the
water-swellable material), more preferably between 70% and 100% by
weight or even from 80% to 100% by weight, and most preferably
between 90% and 100% by weight.
The water-swellable material is typically solid; this includes
gels, flakes, fibers, agglomerates, large blocks, granules and
particles, spheres and other forms known in the art for
superabsorbent or water-swellable polymers described herein.
Preferably, the material is in the form of particles having a mass
median particle size between 10 .mu.m and 1 mm, preferably between
100 .mu.m and 800 .mu.m, as can for example be measured by the
method set out in for example EP-A-0691133.
In one embodiment of the invention the water swellable material of
the invention is in the form of (free flowing) particles with
particle sizes between 10 .mu.m and 1200 .mu.m or even between 50
.mu.m and 800 .mu.m and a mass median particle size between 100 and
600 .mu.m.
In addition, or in another embodiment of the invention, the water
swellable material comprises particles that are essentially
spherical.
In yet another preferred embodiment of the invention the water
swellable material of the invention has a relatively narrow range
of particle sizes with the majority of particles having a particle
size between 50 .mu.m and 800 .mu.m, preferably between 100 .mu.m
and 500 .mu.m, and more preferably between 200 .mu.m and 500
.mu.m.
The water-swellable material of the invention preferably comprises
less than 20% by weight of water, or even less than 10% or even
less than 8% or even less than 5%, or even no water. The
water-content of the water-swellable material can be determined by
the EDANA test, number ERT 430.1-99 (February 1999) which involves
drying the water swellable material at 105 Celsius for 3 hours and
determining the moisture content by the weight loss of the water
swellable materials after drying.
Preferred may be that the water-swellable material comprises two
coatings, obtainable by coating the hydrogel forming polymers twice
or more. This may be the same coating agent or a different coating
agent. For example, the coating may be formed by two layers or
coatings of polymeric elastomeric material, as described herein
below, or it may have a first layer or shell of polymeric
elastomeric material and a second layer of an organic or inorganic
powder, such as various salts, silicates, clay, etc.
Especially preferred water swellable materials of the invention
have a high sorption capacity measured by the CCRC test outlined
below, e.g., of 50 g/g or more, or even 60 g/g or even 80 g/g or
even 100 g/g.
Especially preferred water swellable materials of the invention
have a high permeability for liquid such as can be measured by the
SFC test disclosed in U.S. Pat. No. 5,599,335, U.S. Pat. No.
5,562,646 and U.S. Pat. No. 5,669,894 all of which are incorporated
herein by reference.
In addition, especially preferred water swellable materials of the
invention have a high wet porosity (i.e. this means that once an
amount of the water-swellable material of the invention is allowed
to absorb a liquid and swell, it will typically form a (hydro)gel
or (hydro)gelbed, which has thus a certain wet porosity, in
particular compared to the uncoated water-swellable polymers, as
can be measured by the PHL test disclosed in U.S. Pat. No.
5,562,646 which is incorporated herein by reference (if the
water-swellable material or hydrogel forming polymers are to be
tested at different pressures, the weight used in this test should
be adjusted accordingly).
The use of the coating agent preferably increases the wet porosity
of the water-swellable material herein, compared to the uncoated
hydrogel forming polymers; preferably this increase is at least 50%
or even at least 100%, or even at least 150%.
Most preferred water swellable materials made by the process of the
invention have a high absorption capacity such as preferably
measured by the CCRC test outlined below in combination with a high
permeability (SFC) and high wet porosity (increased by the use of
the coating agent).
Hydrogel Forming Polymers
The hydrogel-forming polymers herein are preferably solid,
preferably in the form of particles, flakes, fibers, agglomerated
particles; most preferably, the polymers are particles having a
mass median particle size as specified above for the
water-swellable material, but slightly increased by the caliper of
the coating as described herein.
As used herein, the term "hydrogel forming polymer" and "coated
hydrogel forming polymer" refers to a polymer which is
substantially water-insoluble, water-swellable and water-gelling,
forming a hydrogel, and which has typically a Cylinder Centrifuge
Retention Capacity (CCRC) as defined below of at least 8 g/g, or
even at least 10 g/g/or even at least 20 g/g. These polymers are
often also referred to in the art as (super-) absorbent polymers
(SAP) or absorbent gelling materials (AGM).
These polymers are typically (lightly) crosslinked polymers,
preferably lightly crosslinked hydrophilic polymers. While these
polymers may in general be non-ionic, cationic, zwitterionic, or
anionic, the preferred polymers are cationic or anionic. Especially
preferred are acid polymers, which contain a multiplicity of acid
functional groups such as carboxylic acid groups, or their salts,
preferably sodium salts. Examples of acid polymers suitable for use
herein include those which are prepared from polymerizable,
acid-containing monomers, or monomers containing functional groups
which can be converted to acid groups after polymerization. Such
monomers include olefinically unsaturated carboxylic acids and
anhydrides, and mixtures thereof. The acid polymers can also
comprise polymers that are not prepared from olefinically
unsaturated monomers.
Examples of such polymers also include polysaccharide-based
polymers such as carboxymethyl starch and carboxymethyl cellulose,
and poly (amino acid) based polymers such as poly (aspartic acid).
For a description of poly (amino acid) absorbent polymers, see, for
example, U.S. Pat. No. 5,247,068, issued Sep. 21, 1993 to Donachy
et al.
Some non-acid monomers can also be included, usually in minor
amounts, in preparing the absorbent polymers herein. Such non-acid
monomers can include, for example, monomers containing the
following types of functional groups: carboxylate or sulfonate
esters, hydroxyl groups, amide-groups, amino groups, nitrile
groups, quaternary ammonium salt groups, and aryl groups (e.g.,
phenyl groups, such as those derived from styrene monomer). Other
optional non-acid monomers include unsaturated hydrocarbons such as
ethylene, propylene, 1-butene, butadiene, and isoprene. These
non-acid monomers are well-known materials and are described in
greater detail, for example, in U.S. Pat. No. 4,076,663 (Masuda et
al.), issued Feb. 28, 1978, and in U.S. Pat. No. 4,062,817
(Westerman), issued Dec. 13, 1977.
Olefinically unsaturated carboxylic acid and anhydride monomers
useful herein include the acrylic acids typified by acrylic acid
itself, methacrylic acid, .alpha.-chloroacrylic acid,
a-cyanoacrylic acid, .beta.-methylacrylic acid (crotonic acid),
.alpha.-phenylacrylic acid, .beta.-acryloxypropionic acid, sorbic
acid, .alpha.-chlorosorbic acid, angelic acid, cinnamic acid,
p-chlorocinnamic acid, .beta.-stearylacrylic acid, itaconic acid,
citroconic acid, mesaconic acid, glutaconic acid, aconitic acid,
maleic acid, fumaric acid, tricarboxyethylene, and maleic
anhydride.
Preferred hydrogel forming polymers contain carboxyl groups, such
as the above-described carboxylic acid/carboxylate containing
groups. These polymers include hydrolyzed starch-acrylonitrile
graft copolymers, partially neutralized hydrolyzed
starch-acrylonitrile graft copolymers, starch-acrylic acid graft
copolymers, partially neutralized starch-acrylic acid graft
copolymers, hydrolyzed vinyl acetate-acrylic ester copolymers,
hydrolyzed acrylonitrile or acrylamide copolymers, slightly network
crosslinked polymers of any of the foregoing copolymers,
polyacrylic acid, and slightly network crosslinked polymers of
polyacrylic acid. These polymers can be used either solely or in
the form of a mixture of two or more different polymers. Examples
of these polymer materials are disclosed in U.S. Pat. No.
3,661,875, U.S. Pat. No. 4,076,663, U.S. Pat. No. 4,093,776, U.S.
Pat. No. 4,666,983, and U.S. Pat. No. 4,734,478.
Most preferred polymer materials used for making the water-sellable
polymers herein are polyacrylates/acrylic acids and derivatives
thereof, preferably (slightly) network crosslinked polymers
partially neutralized polyacrylic acids and/or -starch derivatives
thereof.
Preferred may be that partially neutralized polymeric acrylic acid
is used in the process herein.
The hydrogel forming polymers useful in the present invention can
be formed by any polymerization and/or crosslinking techniques.
Typical processes for producing these polymers are described in
U.S. Reissue Pat. No. 32,649 (Brandt et al.), issued Apr. 19, 1988,
U.S. Pat. No. 4,666,983 (Tsubakimoto et al.), issued May 19, 1987,
and U.S. Pat. No. 4,625,001 (Tsubakimoto et al.), issued Nov. 25,
1986; U.S. Pat. No. 5,140,076 (Harada); U.S. Pat. No. 6,376,618 B1,
U.S. Pat. No. 6,391,451 and U.S. Pat. No. 6,239,230 (Mitchell);
U.S. Pat. No. 6,150,469 (Harada). Crosslinking can be affected
during polymerization by incorporation of suitable crosslinking
monomers. Alternatively, the polymers can be crosslinked after
polymerization by reaction with a suitable reactive crosslinking
agent. Surface crosslinking of the initially formed polymers is a
preferred way to control to some extends the absorbent capacity,
porosity and permeability.
The hydrogel forming polymers may also be surface-crosslinked,
prior to, simultaneously with or after the coating step of the
process herein. Suitable general methods for carrying out surface
crosslinking of absorbent polymers according to the present
invention are disclosed in U.S. Pat. No. 4,541,871 (Obayashi),
issued Sep. 17, 1985; published PCT application WO92/16565
(Stanley), published Oct. 1, 1992, published PCT application
WO90/08789 (Tai), published Aug. 9, 1990; published PCT application
WO93/05080 (Stanley), published Mar. 18, 1993; U.S. Pat. No.
4,824,901 (Alexander), issued Apr. 25, 1989; U.S. Pat. No.
4,789,861 (Johnson), issued Jan. 17, 1989; U.S. Pat. No. 4,587,308
(Makita), issued May 6, 1986; U.S. Pat. No. 4,734,478
(Tsubakimoto), issued Mar. 29, 1988; U.S. Pat. No. 5,164,459
(Kimura et al.), issued Nov. 17, 1992; published German patent
application 4,020,780 (Dahmen), published Aug. 29, 1991; U.S. Pat.
No. 5,140,076 (Harada); U.S. Pat. No. 6,376,618 B1, U.S. Pat. No.
6,391,451 and U.S. Pat. No. 6,239,230 (Mitchell); U.S. Pat. No.
6,150,469 (Harada); and published European patent application
509,708 (Gartner), published Oct. 21, 1992.
Most preferably, the polymers comprise from about 50% to 95% (mol
percentage), preferably about 75% neutralized, (slightly) network
crosslinked, polyacrylic acid (i.e., poly (sodium acrylate/acrylic
acid)). Network crosslinking renders the polymer substantially
water-insoluble and, in part, determines the absorptive capacity
and extractable polymer content characteristics of the absorbent
polymers. Processes for network crosslinking these polymers and
typical network crosslinking agents are described in greater detail
in U.S. Pat. No. 4,076,663.
While the hydrogel forming polymer is preferably of one type (i.e.,
homogeneous), mixtures of hydrogel forming polymers can also be
used in the present invention. For example, mixtures of
starch-acrylic acid graft copolymers and slightly network
crosslinked polymers of polyacrylic acid can be used in the present
invention. Mixtures of (coated) polymers with different physical
properties, and optionally also different chemical properties,
could also be used, e.g., different mean particle size, absorbent
capacity, absorbent speed, SFC value) such as for example disclosed
in U.S. Pat. No. 5,714,156 which is incorporated herein by
reference.
The hydrogel forming polymers preferably have a low amount of
extractables, preferably less than 15% (by weight of the polymers;
1 hour test value), more preferably less than 10% and most
preferably less than 5% of extractables, or even less than 3%. The
extractables and levels thereof and determination thereof are
further described in for example U.S. Pat. No. 5,599,335; U.S. Pat.
No. 5,562,646 or U.S. Pat. No. 5,669,894.
Coating Agents and Polymeric Elastomeric Material
The coating agent herein comprises an elastomeric polymeric
material. It is believed that the elastomeric polymeric materials
provide a return force when being extended and thus enable the
coating (shell/layer) to provide tangential forces around the
hydrogel forming polymer, thereby thus acting like the elastic
membrane of a balloon and providing a resistance to deformation for
the water swellable material of the invention.
Preferred polymeric elastomeric materials herein have a glass
transition temperature Tg of below 38.degree. C., preferably less
than 20.degree. C., more preferably less than 0.degree. C., and
most preferably between 0.degree. C. and -60.degree. C. (i.e., Tg's
before curing).
The coating agent is preferably such that the resulting coating on
the hydrogel forming polymers herein is water-permeable, but not
water-soluble and, preferably not water-dispersible. The water
permeability of the coating should be high enough such that the
coated water swellable material has a sufficiently high free swell
rate as defined above, preferably a free swell rate (FSR) of at
least 0.05 g/g/sec, preferably at least 0.1 g/g/sec, and more
preferably at least 0.2 g/g/sec.
Preferred elastomeric, polymeric materials herein include natural
or synthetic elastomeric polymeric materials, preferably
elastomeric polymeric material selected from the group of natural
rubber, synthetic rubber and thermoplastic elastomeric polymers
that are elastic at 35.degree. C., or below any of the temperatures
above.
Preferred coating agents of the present invention comprise polymers
that form a film by any film forming method known in the art, e.g.,
when being applied (as a spray) from a solution, dispersion or as
hotmelt, for example under the process conditions described below.
Further preferred are polymers that form elastomeric films that are
not tacky or sticky in the dry state. Especially preferred are
coating agents that are not tacky or sticky in the dry state but
are sticky or tacky in the wet state.
The elastomeric polymers useful in coating agents of the present
invention are preferably polymers that can be self-crosslinking,
i.e., form covalent crosslinks in the polymer network to make it
thermoset. Alternatively, crosslinking agents may be added to the
polymers to cause crosslinking after activation, e.g., with high
temperature, described hereinafter under the discussion of the
curing step c).
In a preferred embodiment, the elastomeric polymers useful in
coating agents of the present invention may be reactive with the
water-swellable polymers, preferably thereto being a carboxylated
elastomeric polymeric (elastomeric) material.
Especially preferred coating agents comprise polymers, co-polymers,
and/or blockcopolymers of ethylene, vinyl compounds (e.g., styrene,
vinylacetate, vinylformamide), polyunsaturated monomers (e.g.,
butadiene, isoprene), as well as polyurethanes, polyethers,
polydimethylsiloxanes, proteins, which may optionally be grafted
and/or be partially modified with chemical substituents (e.g.,
hydroxyl groups or carboxylates).
Highly preferred materials useful in the coating agent herein are
commercially available elastomeric latex materials, such for
example from the Hystretch, Vinamul, Dur-O-Set Elite, GenFlo and
AcryGen series, in particular Hystretch V43, Hystretch V60,
Hystretch V23, Vinamul 3301, Vinamule Dur-O-Set Elite Ultra,
Vinamul Dur-O-Set Elite 21, Rovene 4151, Rovene 5550, GenFlo 3075,
GenFlo 3088, GenFlo 3000, Suncryl CP-75, AcryGen DV242DX, AcryGen
1900 D.
Hystretch is a trademark of Noveon Inc., 9911 Brecksville Road,
Cleveland, Ohio 44141-3247, USA. Vinamul and Dur-O-Set Elite are
trademarks of Vinamul Polymers, De Asselen Kuil 20, 6161 RD Geleen,
NL. Rovene is a trademark of Mallard Creek Polymers, 14700 Mallard
Creek Road, Charlotte, N.C. 28262, USA. GenFlo, AcryGen and Suncryl
are trademarks of Omnova Solutions Inc., 2990 Gilchrist Road,
Akron, Ohio 44305-4418, USA.
Particularly preferred coating agents comprise Surface Hydrophilic
Elastic Latexes (SHEL) as described for example in U.S. Pat. No.
4,734,445; U.S. Pat. No. 4,835,211, U.S. Pat. No. 4,785,030; EP 0
799 258 B1 all of which are incorporated herein by reference. These
particularly preferred SHEL materials typically comprise: (1) a
liquid phase selected from the group consisting of water,
water-miscible solvents and mixtures thereof; and (2) an effective
amount of latex particles dispersed in the liquid phase. These
particles comprise an elastomeric hydrophobic core and an outer
hydrophilic shell integral with the elastomeric core. The
hydrophilic shell of the particles ultimately translates into the
hydrophilic surface of films formed therefrom, and also
advantageously stabilizes the particles as colloids in the liquid
phase. The shell comprises hydrophilic moieties -X which are
attached to the core via linking group L-. When the liquid phase is
removed, the particles form an elastomeric film having a
substantially permanent hydrophilic surface. The SHEL compositions
have the desirable property of forming elastomeric films having a
hydrophilic surface and surface hydrophilicity, combined with other
properties such as flexibility, elasticity and strength.
Other examples of polymeric elastomeric materials include materials
with elastic properties like VFE-CD, available from Tredegar, and
L-86, available from Fulflex (Limerick, Ireland), or preferably
L-89, available from Fulflex, or more preferred are of course one
or more of these materials itself.
Also mixtures of elastomeric polymeric materials may be present in
the coating agent.
The coating agent may also comprise other components, including the
following.
Preferred polymeric elastomeric materials for use in the coating
agent herein are strain hardening and/or strain crystallizing.
While there are some elastomeric polymers that are strain
crystallizing, this property can also be imparted by the addition
or blending of materials into the polymer. Hereto, the coating
agent may comprise additional components that increase the strain
hardening and/or strain crystallization of the elastomeric
polymeric material, such as organic or inorganic fillers.
Nonlimiting examples of inorganic fillers include various
water-insoluble salts, and other (preferably nanoparticulate)
materials such as for example chemically modified silica, also
called active or semi-active silica that are for example available
as fillers for synthetic rubbers. Examples for such fillers are
UltraSil VN3, UltraSil VN3P, UltraSil VN2P, and UltraSil 7000GR
available from Degussa AG, Wei.beta.frauenstra.beta.e 9, D-60287
Frankfurt am Main, Germany.
The coating agent is preferably hydrophilic and in particular
surface hydrophilic. The surface hydrophilicity may be determined
by methods known to those skilled in the art. In a preferred
execution, the hydrophilic coating agents or elastomeric polymeric
materials are materials that are wetted by the liquid that is to be
absorbed (0.9% saline; urine). They may be characterized by a
contact angle that is less than 90 degrees. Contact angles can for
example be measured with the Video-based contact angle measurement
device, Kruss G10-G1041, available from Kruess, Germany or by other
methods known in the art.
It may also be preferred that the resulting water-swellable
material or coated hydrogel forming polymer particles are
hydrophilic. This hydrophilicity may be measured as described in
co-pending U.S. patent application Ser. No. 10/881,090.
If the coating agent itself is not hydrophilic, it can be made
hydrophilic for example by treating it with surfactants, plasma
coating, plasma polymerization, or other hydrophilic surface
treatments as known to those skilled in the art.
Preferred compounds to be added to make the hydrophilic coating
agent, or subsequently to be added to the resulting coated hydrogel
forming polymers are for example:
N-(2-Acetamido)-2-aminoethansulfonic-acid,
N-(2-Acetamido)-imino-di-acetic-acid, N-acetyl-glycin,
.beta.-Alanin, Aluminum-hydroxy-acetat, N-Amidino-glycin,
2-Amino-ethyl-hydrogenphosphate, 2-Amino-ethyl-hydrogensulfate,
Amino-methan-sulfonic-acid, Maleinic-acid, Arginin,
Asparaginic-acid, Butane-di-acid, Bis(1-aminoguanidinium)-sulfat,
2-Oxo-propionic-acid, Tri-Calcium-di-citrat, Calciumgluconat,
Calcium-saccharat, Calcium-Titriplex.RTM., Carnitin, Cellobiose,
Citrullin, Creatin, Dimethylaminoacetic acid,
THAM-1,2-disulfonic-acid, Ethylendiammoniumsulfate, Fructose,
Fumaric-acid, Galactose, Glucosamine, Gluconic-acid, Glutamine,
2-Amino-glutaric-acid, Glutaric-acid, Glycin, Glycylglycin,
Imino-di-acetic-acid, Magnesium-glycerophosphate, Oxalicacid,
Tetrahydroxy-adipinic-acid, Taurin, N-Methyl-taurin,
Tris-(hydroxymethyl)-aminomethan,
N-(Tris-(hydroxymethyl)-methyl)-2-aminoethansulfonicacid.
Alternatively, the coating agent can be made hydrophilic with a
hydrophilicity boosting composition comprising a
hydrophilicity-boosting amount of nanoparticles. By hydrophilicity
boosting amount, it is intended that an amount of nanoparticles be
present in the hydrophilicity boosting compositions, which are
sufficient to make a substrate to which it is applied more
hydrophilic. Such amounts are readily ascertained by one of
ordinary skill in the art; it is based on many factors, including
but not limited to, the substrate used, the nanoparticles used, the
desired hydrophilicity of the resulting coated water-swellable
material.
Nanoparticles are particles that have a primary particle size
(diameter), which is in the order of magnitude of nanometers. That
is, nanoparticles have a particle size ranging from about 1 to
about 750 nanometers. Nanoparticles with particle sizes ranging
from about 2 nm to about 750 nm can be economically produced.
Non-limiting examples of particle size distributions of the
nanoparticles are those that fall within the range from about 2 nm
to less than about 750 nm, alternatively from about 2 nm to less
than about 200 nm, and alternatively from about 2 nm to less than
about 150 nm.
The particle size of the nanoparticles is the largest diameter of
the nanoparticle and may be measured by any method known to the
skilled person.
The mean particle size of various types of nanoparticles may differ
from the individual particle size of the nanoparticle. For example,
a layered synthetic silicate can have a mean particle size of about
25 nanometers while its particle size distribution can generally
vary between about 10 nm to about 40 nm. (It should be understood
that the particle sizes that are described herein are for particles
when they are dispersed in an aqueous medium and the mean particle
size is based on the mean of the particle number distribution.
Non-limiting examples of nanoparticles can include crystalline or
amorphous particles with a particle size from about 2 to about 750
nanometers. Boehmite alumina can have an average particle size
distribution from 2 to 750 nm.).
If the hydrophilicity boosting composition does not consist of the
nanoparticles, but comprises other ingredients, then it is
preferred that the nanoparticles are present in the hydrophilicity
boosting compositions, or when added to the coating agent, at
levels of from about 0.0001% to about 50%, preferably from about
0.001% to about 20% or even to 15%, and more preferably from about
0.001% to about 10%, by weight of the composition or the coating
agent.
Either organic or inorganic nanoparticles may be used in the
hydrophilicity boosting composition; inorganic nanoparticles are
preferred. Inorganic nanoparticles generally exist as oxides,
silicates, carbonates and hydroxides. Some layered clay minerals
and inorganic metal oxides can be examples of nanoparticles. The
layered clay minerals suitable for use in the present invention
include those in the geological classes of the smectites, the
kaolins, the illites, the chlorites, the attapulgites and the mixed
layer clays. Typical examples of specific clays belonging to these
classes are the smectices, kaolins, illites, chlorites,
attapulgites and mixed layer clays. Smectites, for example, include
montmorillonite, bentonite, pyrophyllite, hectorite, saponite,
sauconite, nontronite, talc, beidellite, volchonskoite. Kaolins
include kaolinite, dickite, nacrite, antigorite, anauxite,
halloysite, indellite and chrysotile. Illites include bravaisite,
muscovite, paragonite, phlogopite and biotite and vermiculite.
Chlorites include corrensite, penninite, donbassite, sudoite,
pennine and clinochlore. Attapulgites include sepiolite and
polygorskyte. Mixed layer clays include allevardite and
vermiculitebiotite. Variants and isomorphic substitutions of these
layered clay minerals offer unique applications.
Layered clay minerals may be either naturally occurring or
synthetic. An example of one non-limiting embodiment of the coating
composition uses natural or synthetic hectorites, montmorillonites
and bentonites. Another embodiment uses the hectorites clays
commercially available, and typical sources of commercial
hectorites are the LAPONITEs.TM. from Southern Clay Products, Inc.,
U.S.A; Veegum Pro and Veegum F from R. T. Vanderbilt, U.S.A.; and
the Barasyms, Macaloids and Propaloids from Baroid Division,
National Read Comp., U.S.A.
In one preferred embodiment of the present invention the
nanoparticles comprise a synthetic hectorite a lithium magnesium
silicate. One such suitable lithium magnesium silicate is
LAPONITE.TM., which has the formula:
[Mg.sub.wLi.sub.xSi.sub.8O.sub.20OH.sup.4-yF.sub.y].sup.z- wherein
w=3 to 6, x=0 to 3, y=0 to 4, z=12-2w-x, and the overall negative
lattice charge is balanced by counter-ions; and wherein the
counter-ions are selected from the group consisting of selected
Na.sup.+, K.sup.+, NH.sub.4.sup.+, Cs.sup.+, Li.sup.+, Mg.sup.++,
Ca.sup.++, Ba.sup.++, N(CH.sub.3).sub.4.sup.+ and mixtures thereof.
(If the LAPONITE.TM. is "modified" with a cationic organic
compound, then the "counter-ion" could be viewed as being any
cationic organic group (R.sup.+).)
Other suitable synthetic hectorites include, but are not limited to
isomorphous substitutions of LAPONITE.TM., such as, LAPONITE B.TM.,
LAPONITE S.TM., LAPONITE XLS.TM., LAPONITE RD.TM., LAPONITE
XLG.TM., and LAPONITE RDS.TM..
The nanoparticles may also be other inorganic materials, including
inorganic oxides such as, but not limited to, titanium oxide
silica, zirconium oxide, aluminum oxide, magnesium oxide and
combinations thereof. Other suitable inorganic oxides include
various other inorganic oxides of alumina and silica.
In one preferred embodiment of the present invention the
nanoparticles comprise a Boehmite alumina ([Al(O)(OH)].sub.n) which
is a water dispersible, inorganic metal oxide that can be prepared
to have a variety of particle sizes or range of particle sizes,
including a mean particle size distribution from about 2 nm to less
than or equal to about 750 nm. For example, a boehmite alumina
nanoparticle with a mean particle size distribution of around 25 nm
under the trade name Disperal P2.TM. and a nanoparticle with a mean
particle size distribution of around 140 nm under the trade name of
Dispal.RTM. 14N4-25 are available from North American Sasol,
Inc.
In one preferred embodiment of the present invention the
nanoparticles are selected from the group consisting of titanium
dioxide, Boehmite alumina, sodium magnesium lithium fluorosilicates
and combinations thereof.
Use of mixtures of nanoparticles in the hydrophilicity boosting
compositions is also within the scope of the present invention.
The hydrophilicity boosting compositions of the present invention
may also include optional ingredients such as, a carrier,
surfactant and other adjunct ingredients. Suitable carriers include
liquids, solids and gases. One preferred carrier is water, which
can be distilled, deionized, or tap water. Water is valuable due to
its low cost, availability, safety, and compatibility.
Optionally, in addition to or in place of water, the carrier can
comprise a low molecular weight organic solvent. Preferably, the
solvent is highly soluble in water, e.g., ethanol, methanol,
acetone, ethylene glycol, propanol, isopropanol, and the like, and
mixtures thereof. Low molecular weight alcohols can reduce the
surface tension of the nanoparticle dispersion to improve
wettability of the substrate. This is particularly helpful when the
substrate is hydrophobic. Low molecular weight alcohols can also
help the treated substrate to dry faster. The optional water
soluble low molecular weight solvent can be used at any suitable
level. The carrier can comprise any suitable amount of the
composition, including but not limited to from about 10% to about
99%, alternatively from about 30% to about 95%, by weight of the
coating composition.
The hydrophilicity boosting composition may also comprise organic,
e.g., latex nanoparticles, so-called nanolatexes. A "nanolatex", as
used herein, is a latex with a particle size less than or equal to
about 750 nm. A "latex" is a dispersion of water-insoluble polymer
particles that are usually spherical in shape. Nanolatexes may be
formed by emulsion polymerization. "Emulsion polymerization" is a
process in which monomers of the latex are dispersed in water using
a surfactant to form a stable emulsion followed by polymerization.
Particles are typically produced which can range in size from about
2 to about 600 nm. When the nanolatexes are elastomeric polymers,
then they are considered coating agents for the purpose of the
invention, and not (part of) a hydrophilicity boosting
compositions.
Surfactants are especially useful in the coating composition as
wetting agents to facilitate the dispersion of nanoparticles onto
the substrate. Surfactants are preferably included when the coating
composition is used to treat a hydrophobic substrate.
Suitable surfactants can be selected from the group including
anionic surfactants, cationic surfactants, nonionic surfactants,
amphoteric surfactants, ampholytic surfactants, zwitterionic
surfactants and mixtures thereof. Nonlimiting examples of
surfactants useful in the compositions of the present invention are
disclosed in McCutcheon's, Detergents and Emulsifiers, North
American edition (1986), published by Allured Publishing
Corporation; McCutcheon's, Functional Materials, North American
Edition (1992); U.S. Pat. Nos. 5,707,950 and 5,576,282; and U.S.
Pat. No. 3,929,678, to Laughlin et al., issued Dec. 30, 1975.
When a surfactant is used in the coating agent, it may be added at
an effective amount to provide facilitate application of the
coating composition. Surfactant, when present, is typically
employed in compositions at levels of from about 0.0001% to about
60%, preferably from about 0.001% to about 35%, and more preferably
from about 0.001% to about 25%, by weight of the coating agent.
Nonlimiting examples of surfactants include nonionic and amphoteric
surfactants such as the C.sub.12-C.sub.18 alkyl ethoxylates ("AE")
including the so-called narrow peaked alkyl ethoxylates and
C.sub.6-C.sub.12 alkyl phenol alkoxylates (especially ethoxylates
and mixed ethoxy/propoxy), C.sub.12-C.sub.18 betaines and
sulfobetaines ("sultaines"), C.sub.10-C.sub.18 amine oxides, and
the like. Another class of useful surfactants is silicone
surfactants and/or silicones. They can be used alone and/or
alternatively in combination with the alkyl ethoxylate surfactants
described herein. Nonlimiting examples of silicone surfactants are
the polyalkylene oxide polysiloxanes having a dimethyl polysiloxane
hydrophobic moiety and one or more hydrophilic polyalkylene side
chains, and having the general formula:
R.sup.1--(CH.sub.3).sub.2SiO--[(CH.sub.3).sub.2SiO].sub.a--[(CH.sub.3)(R.-
sup.1)SiO].sub.b--Si(CH.sub.3).sub.2--R.sup.1 wherein a+b are from
about 1 to about 50, and each R.sup.1 is the same or different and
is selected from the group consisting of methyl and a
poly(ethyleneoxide/propyleneoxide) copolymer group having the
general formula:
--(CH.sub.2).sub.nO(C.sub.2H.sub.4O).sub.c(C.sub.3H.sub.6O).sub.- d
R.sup.2, wherein n is 3 or 4; total c (for all polyalkyleneoxy side
groups) has a value of from 1 to about 100, alternatively from
about 6 to about 100; total d is from 0 to about 14; alternatively
d is 0; total c+d has a value of from about 5 to about 150,
alternatively from about 9 to about 100 and each R.sup.2 is the
same or different and is selected from the group consisting of
hydrogen, an alkyl having 1 to 4 carbon atoms, and an acetyl group,
alternatively hydrogen and methyl group. Each polyalkylene oxide
polysiloxane has at least one R.sup.1 group being a
poly(ethyleneoxide/propyleneoxide) copolymer group. Silicone
superwetting agents are available from Dow Corning as silicone
glycol copolymers (e.g., Q2-5211 and Q2-5212).
The coating agent is preferably applied in fluid form, e.g., as
melt (or so-called hotmelt), solution or dispersion. Preferred are
water-based solutions or dispersions. In the context of this
invention and as it is typically used in the art, the latexes
referred herein are thus typically applied as water based
dispersions of specific latex polymers, whereby the polymeric latex
particles--typically of spherical shape--are suspended or dispersed
(stable) in a water based liquid.
Thus, the coating agent may also comprise a solvent or dispersing
liquid, such as water, THF (tetrahydrofurane), cyclohexane or other
solvents or dispersing liquids that are able to dissolve or
disperse the elastomeric polymer and subsequently can be evaporated
such as to form a (dry) coating shell or layer.
As it is known to those skilled in the art, in particular for latex
dispersions with lower amounts of the polymer in the water
dispersion, the viscosity is decreased, which enables good
spreading of the coating agent. On the other hand, it is preferred
to have higher amounts of polymer in the water dispersion to aid
film quality and coalescence, and to minimize the amount of liquid
that needs to be dried-off or evaporated. Thus, the skilled person
would know how to select a high enough but not to low concentration
to obtain the desired coating.
Preferably, the coating agent comprises from 0% to 95% by weight of
a dispersing liquid or solvent, such as water. Preferred is that
the coating agent comprises at least 10% by weight (of the coating
agent) of the polymeric elastomeric material, more preferably from
20% to 80% or even from 30% to 70%, the remaining percentage being
said liquid and/or fillers/hydrophilicity aids, spreading aids,
etc., as described herein.
Process of the Invention for Making the Solid Water-Swellable
Material
The water-swellable material of the inventions may be made by any
known process.
A preferred coating process for coating the hydrogel forming
polymers herein involves: a) obtaining hydrogel forming polymers;
and b) simultaneously with or subsequently to step a), applying a
coating agent comprising an elastomeric polymeric material to at
least part of said hydrogel forming polymers to obtain coated
hydrogel forming polymers;
and preferably c) prior to, simultaneous with or subsequent to step
b), obtaining said hydrogel forming polymers or coated hydrogel
forming polymers in solid, preferably particulate, form.
In step a) `obtaining` the hydrogel forming polymers, as described
herein above, includes using commercially available hydrogel
forming polymers, or forming the hydrogel forming polymers by any
known process from precursors. It includes also for example the
possibility that step a) and b) are done simultaneously and that
step a) involves reacting the relevant polymer precursors to form
the hydrogel forming polymer in the same reaction conditions/medium
as the coating step b) (for example, the polymer precursors and
coating agent can be mixed together).
It should be noted that optional process steps may take place prior
to, or simultaneous with step a) and/or b) and/or c), such as that
the hydrogel forming polymer may be surface crosslinked prior to
step b) or that the coating agent or hydrogel forming polymers may
be submitted to a hydrophilic treatment, to render them more
hydrophilic, prior to step b).
The coating step b) may be done by any known method, for example by
mixing or dispersing the hydrogel forming polymers (or precursors
thereto) in the coating agent or melt or solution or dispersion
thereof; by spraying the coating agent or (hot) melt, solution or
dispersion thereof onto the polymers; by introducing the coating
agent, or melt, dispersion or solution thereof, and the hydrogel
forming polymers (or precursors thereto) in a fluidised bed or
Wurster coater; by agglomerating the coating agent, or melt,
solution or dispersion thereof, and the hydrogel forming polymers
(or precursors thereof); by dip-coating the (particulate) hydrogel
forming polymers in the coating agent, melt, dispersion or solution
thereof. Other suitable mixers include for example twin drum
mixers, so called "Zig-Zag" mixers, horizontally operating
plough-share mixers, Lodige mixers, cone screw mixers, or
perpendicularly cylindrical mixers having coaxially rotating
blades. Examples of preferred coating processes are for example
described in U.S. Pat. No. 5,840,329 and U.S. Pat. No.
6,387,495.
In an alternative embodiment of the invention, the coating step b)
may be done by applying the coating agent in the form of a foam,
preferably in the form of an open-cell foam, leading to a porous
coating. In yet an alternative embodiment the coating step may be
done by forming a fibrous network on the surface of the hydrogel
forming polymers such as for example by applying the coating agent
in the form of meltblown microfibers, such that an essentially
connected coating is formed (as described herein).
In a yet another embodiment, the coating step b) may be done by
applying a coating agent that comprises polymerizable material,
polymerizable into elastomeric polymeric material (such as the
monomers of such polymeric material, as described herein) and
directly polymerising these on the surface of the hydrogel forming
polymers.
The coating agents may also comprise solvents such as organic or
optionally water-miscible solvents. Suitable organic solvents are,
for example, aliphatic and aromatic hydrocarbons, alcohols, ethers,
esters, and ketones. Suitable water-miscible solvents are, for
example, aliphatic alcohols, polyhydric alcohols, ethers, and
ketones.
If the coating agent is in the form of a latex dispersion, it may
be further preferred to add processing aids (such for example
coalescing aids) subsequently or prior to the coating step b) in
order to aid a good film formation of the coating layer.
The process may comprise a curing step (d) which typically results
in a further strengthened or more continuous or more completely
connected coating. For example, during the curing step the coating
may be annealed or cross-linked, as described below in more
detail.
The curing step may be done by any known method. Typically, the
curing step involves a heat treatment of the resulting coated
hydrogel forming polymers; it may be done by for example radiation
heating, oven heating, convection heating, or placing the coated
polymers under vacuum and increased temperature, azeotropic
heating, and it may for example take place in conventional
equipment used for drying, such as fluidized bed driers.
Preferred may be that a vacuum is applied as well or that the
curing or drying is done under an inert gas (to avoid
oxidation).
Preferably, the heat treatment involves heating the coated hydrogel
forming polymers at a temperature of at least 70.degree. C., or
even at least 80.degree. C., or even at least 100.degree. C., or
even at least 120.degree. C. or even at least 130.degree. C. or
even at least 140.degree. C., and preferably for at least 5
minutes, or even for at least 10 minutes or even for at least 15
minutes, or even at least 30 minutes or even at least 1 hour or
even at least 2 hours. Preferred is that the maximum temperature is
up to 300.degree. C., or even up to 250.degree. C. or even up to
200.degree. C.
This heat-treatment may be done once, or it may be repeated, for
example the heat treatment may be repeated with different
temperatures, for example first at a lower temperature, for example
from 70.degree. C. or 80.degree. C. to 100.degree. C., as described
above, for example for at least 1 hour, and subsequently at a
higher temperature, for example 120-140.degree. C. or even up to
300.degree. C., for at least 10 minutes, to invoke chemical
reactions, such as further polymerising or cross-linking the
wet-extensible polymers of the coating agent.
In one preferred embodiment, the drying step may be done first at a
temperature of 80-100.degree. C. for any time, preferably at least
1 hour and preferably up to 48 hours, and the curing step
(involving for example annealing/cross-linking) may be done at a
temperature from 120-300.degree. C. or even from 130-250.degree.
C., or even from 140-200.degree. C., for at least 5 minutes, or
even at least 15 minutes or even at least 30 minutes, or preferably
at least 1 hour, preferably up to 4 hours or even up to 12
hours.
During the curing step, the coated hydrogel forming polymers may
also be dried at the same time, but in a preferred embodiment, the
coated hydrogel forming polymers are submitted to a separate drying
step, prior to the coating step, which involves any of the
treatments described above as curing treatment, or preferably a
vacuum treatment or heat treatment at a temperature below the
curing temperatures above, and typically for a time period which is
longer than the curing time.
Preferably, when the coating agent is a film-forming agent or
comprises a film forming elastomeric material, the curing and/or
drying temperature is typically above the minimum film forming
temperature (MFFT) of the coating agent or material thereof.
The resulting water-swellable material is preferably solid and
thus, if the hydrogel forming polymers of step a) or the resulting
coated polymers of step b) are not solid, a subsequent process step
is required to solidify the resulting coated polymers of step b),
e.g., a so-called solidifying or preferably particle forming step,
as known in the art. This may preferably be done prior to, or
simultaneously with step c).
The solidifying step includes for example drying the hydrogel
forming polymers and/or the coated polymers of step b) (e.g., if
the step b) involve a dispersion, suspension or solution of any of
the ingredients) by increasing the temperature and/or applying a
vacuum, as described herein. This may be simultaneously with, or
occur automatically with the curing step c). The solidifying step
may also include a cooling step, if for example a melt is used.
Subsequently, any known particle forming process may also be used
here for, including agglomeration, extrusion, grinding and
optionally followed by sieving to obtain the required particle size
distribution.
The inventors found another preferred way to provide coatings with
elastomeric material on cores of hydrogel forming polymers, namely
by providing a coating that has a significantly larger surface area
than the outer surface area of the hydrogel forming polymer (core),
so that when the polymers swell, the coating can `unfold` and
extend. The inventors found a very easy and convenient way to
provide such coated hydrogel forming polymers, namely by applying
the coating agent on hydrogel forming polymers, which are in
swollen state due to absorption of a liquid (e.g., water), and then
removing the water or part thereof, so that the hydrogel forming
polymers (in the core) shrink again, but the coating maintains its
original surface area. The surface are of the coating is then
larger than the surface area of the polymer core, and the coating
is then typically wrinkled; it can unwrinkle when the hydrogel
forming polymers absorb water and swell, without encountering any
strain/stress on the coating due to the swelling of the hydrogel
forming polymers.
A highly preferred process thus involves the step of obtaining
hydrogel forming polymers and immersing these in a dispersion or
solution of a coating agent containing a liquid (water), such as
the latex dispersions described above, typically under thorough
stirring. The hydrogel forming polymers will absorb the liquid, and
thereby, the elastomeric material of the coating agent (latex
polymer) is automatically `transferred` to the surface of hydrogel
forming polymers (particles). The amount of hydrogel forming
polymers and amount of water and latex can be adjusted such that
the hydrogel forming polymers can absorb about all water present in
the dispersion and that when this is achieved, the hydrogel forming
polymers, coated with the latex, are in the form of a gel "powder".
The resulting coating is typically under zero strain/stress.
The process may also involve addition of further processing aids in
any of the steps, such as granulation aids, flow aids, drying aids.
For some type of coating agents, the coated hydrogel forming
polymers may potentially form agglomerates. Any flow aids known in
the art may be added (for example prior to or during the coating
step, or preferably during the drying and/or annealing and/or
cross-linking step (s), as discussed below), for example Aerosil
200, available from Degussa has been found to be a good flow
aid.
Also, it may be useful to mechanically agitate the coated polymers
during the curing or drying step, such as by stirring.
Highly preferred may be that the process involves addition of a
spreading aid and/or surfactant, which facilitates the coating step
b).
Preferred (Disposable) Absorbent Articles and Structures
The absorbent structure of one embodiment of the invention is
typically for use in disposable absorbent articles, such as
preferably interlabial products, sanitary napkins, panty liners,
and preferably adult incontinent products, baby diapers, nappies
and training pants.
Typically, the absorbent structure of the invention is that part of
an absorbent article which serves to store the bodily fluid, e.g.,
the storage layer of an absorbent article. As known in the art,
this may be in direct contact with an acquisition layer, or in one
embodiment of the invention, it may form a unitary structure with
an acquisition layer. In yet another embodiment of the invention
the absorbent structure is an acquisition layer for use in an
absorbent article.
The absorbent structure may comprise the water-swellable material
of the invention at any weight level or concentration, but
preferably, in particular when the absorbent structure serves as a
storage layer, or when the absorbent structure comprises a layer
that serves as storage layer, the structure or layer comprises
large amounts of the water-swellable material herein, compared to
possible other components of the structure, i.e. preferably more
than 50% or even more than 70% or even more than 80% or even more
than 90% of the water-swellable material herein, by weight of the
structure or (storage) layer thereof.
For example, the water-swellable material may be mixed with
absorbent fibrous material, such as an airfelt material, which can
provide a matrix for immobilization of the water-swellable
material. However, preferably a relatively low amount of absorbent
fibrous (cellulose) material is used in the absorbent structure.
Thus, if the absorbent structure is a liquid storage layer or when
the absorbent structure comprises one or more liquid storage
layers, it may be preferred that said liquid structure or said
liquid storage layer comprises large amounts of the super absorbent
material herein and only very little or no absorbent (cellulose)
fibers, e.g., preferably less than 40% weight of that layer, or
even less than 20% by weight or even less than 10 weight % or even
less than 5% by weight of absorbent fibrous (cellulose) material,
and preferably more than 50% or even more than 70% or even more
than 80% or even more than 90% by weight of the water-swellable
material herein.
The absorbent structure may comprise a wrapping material, which
wraps the portion comprising the water-swellable material, a
so-called core wrap material. In one preferred embodiment the core
wrap material comprises a top layer and a bottom layer, being
furthest away from the skin of the user. The core wrap material,
the top layer or the bottom layer can be provided from a non-woven
material. One preferred material is a so-called SMS material,
comprising a spunbonded, a melt-blown and a further spunbonded
layer. Highly preferred are permanently hydrophilic non-wovens, and
in particular nonwovens with durably hydrophilic coatings. An
alternative preferred material comprises a SMMS-structure. The top
layer and the bottom layer may be provided from two or more
separate sheets of materials or they may be alternatively provided
from a unitary sheet of material.
Preferred non-woven materials are provided from synthetic fibers,
such as PE, PET and most preferably PP. As the polymers used for
nonwoven production are inherently hydrophobic, they are preferably
coated with hydrophilic coatings, e.g., coated with nanoparticles,
as known in the art.
Notably, permanently hydrophilic non-wovens are also useful in
other parts of an absorbent article, for example, as or in the
topsheet.
In a preferred embodiment of the present invention the absorbent
structure comprises a wrapping material, the water-swellable
material, described herein, and a thermoplastic material and/or a
layer of adhesive, which may be a (non-absorbing) fibrous layer of
adhesive.
Preferred absorbent structures can for example be made as follows:
a) providing a substrate material that can serve as a wrapping
material; b) depositing water-swellable material onto a first
surface of the substrate material, preferably in a pattern
comprising at least one zone which is substantially free of
water-swellable material, and the pattern comprising at least one
zone comprising water-swellable material, preferably such that
opening are formed between the separate zones with water-swellable
material; c) depositing a thermoplastic material onto the first
surface of the substrate material and the water-swellable material,
such that portions of the thermoplastic material are in direct
contact with the first surface of the substrate and portions of the
thermoplastic material are in direct contact with the
water-swellable material; d) and then typically closing the above
by folding the substrate material over, or by placing another
substrate matter over the above.
Preferred disposable absorbent article herein are sanitary napkins,
panty liners, adult incontinence products and infant diapers or
training or pull-on pants, whereby articles which serve to absorb
urine, e.g., adult incontinence products, diapers and training or
pull-on pants are most preferred articles herein.
Preferred articles herein have a topsheet and a backsheet, which
each have a front region, back region and crotch region, positioned
therein between. The absorbent structure of the invention is
typically positioned in between the topsheet and backsheet.
Preferred backsheets are vapour pervious but liquid impervious.
Preferred topsheet materials are at least partially hydrophilic;
preferred are also so-called apertured topsheets. Preferred maybe
that the topsheet comprises a skin care composition, e.g., a
lotion.
These preferred absorbent articles typically comprise a liquid
impervious (but preferably gas or water vapour pervious) backsheet,
a fluid pervious topsheet joined to, or otherwise associated with
the backsheet, and the absorbent structure according to the present
invention positioned between the backsheet and the topsheet. Such
articles are well known in the art and fully disclosed in various
documents mentioned throughout the description, e.g., in EP 752
892.
A preferred diaper or training pants herein has a front waist band
and a back waist band, whereby the front waist band and back waist
band each have a first end portion and a second end portions and a
middle portion located between the end portions, and whereby
preferably the end portions comprise each a fastening system, to
fasten the front waist band to the rear waist band or whereby
preferably the end portions are connected to one another, and
whereby the middle portion of the back waist band and/or the back
region of the backsheet and/or the crotch region of the backsheet
comprises a landing member, preferably the landing member
comprising second engaging elements selected from loops, hooks,
slots, slits, buttons, magnets. Most preferred are hooks, adhesive
or cohesive second engaging elements. Preferred may be that the
engaging elements on the article, or preferably diaper are provided
with a means to ensure they are only engage able at certain
moments, for example, they may be covered by a removable tab, which
is removed when the engaging elements are to be engaged and may be
re-closed when engagement is no longer needed, as described
above.
Preferred diapers and training pants herein have one or more sets
of leg elastics and/or barrier leg cuffs, as known in the art.
Preferred may also be that the topsheet has a large opening,
preferably with elastication means along the length thereof, where
through waist material can pass into a void space above the
absorbent structure, and which ensures it is isolated in this void
space, away from the wearer's skin.
Process Examples and Materials Made by the Process
Preparation of Hydrogel Forming Polymers that are Especially Useful
for Use in Process Step a) of the Invention.
EXAMPLE 1.1
Process for Preparation of Spherical Hydrogel Forming Polymer
Particles
Spherical core polymer particles may be obtained by UMSICHT
(Fraunhofer Institut Umwelt-, Sicherheits-, Energietechnik,
Oberhausen, Germany), or made by following the adapted procedure
below:
40 g glacial acrylic acid (AA) is placed into a beaker, and 1712 mg
MethyleneBisAcrylAmide (MBAA) is dissolved in the acid. Separately,
13.224 g solid NaOH is dissolved in 58.228 g water and cooled. The
NaOH solution is then slowly added to the acrylic acid, and the
resulting solution chilled to 4-10.degree. C.
In a second beaker, 400 mg ammoniumperoxodisulfate (APS) and 400 mg
sodiummetabisulfite are mixed and dissolved in 99.2 ml water. This
solution is also chilled to 4-10.degree. C.
With the use of two equal peristaltic pumps, both solutions are
combined and pumped at equal rates through a short static mixer
unit, after which they are dropped as individual droplets into
60-80.degree. C. hot silicone oil (Roth M 50, cat. #4212.2) which
is in a heated, about 2 m long, glass tube. The pump rate is
adjusted such that individual droplets sink through the oil in the
tube, while also avoiding premature polymerization in the mixer
unit. The polymerization proceeds during the descent of the
droplets through the oil, and particles (gelled polymer droplets)
are formed, which can be collected in a heated 1 liter Erlenmeyer
flask attached to the bottom of the tube.
After completion of the addition, the oil is allowed to cool, and
the spheres are collected by draining the oil. Excess oil is
removed by washing with i-propanol, and the particles (spheres) are
pre-dried by exposing them to excess i-propanol for 12-24 hours.
Additional washings with i-propanol may be needed to remove traces
of the silicone oil. The particles (spheres) are then dried in a
vacuum oven at 60-100.degree. C. until a constant weight is
obtained.
The amount of MBAA may be adjusted, depending on what properties
are required from the resulting polymers, e.g., when 0.3 mol % (per
mol AA) MBAA is used, the resulting hydrogel forming polymer
particles have a CCRC of about 50 g/g (absorption of 0.9% saline
solution, as determined by methods known in the art and described
herein); when 1.0 mol % (per mol AA) MBAA is used, the resulting
hydrogel forming polymer particles have a CCRC of about 19 g/g;
when 2.0 mol % (per mol AA) MBAA is used, the resulting hydrogel
forming polymer particles have a CCRC of about 9 g/g.
All compounds were obtained by Aldrich Chemicals, and used without
purification.
EXAMPLE 1.2
Process for the Preparation of Hydrogel Forming Polymers Useful
Herein
To 300 g of glacial acrylic acid (AA), an appropriate amount of the
core crosslinker (e.g., MethyleneBisAcrylAmide, MBAA) is added (see
above) and allowed to dissolve at ambient temperature. A 2500 ml
resin kettle (equipped with a four-necked glass cover closed with
septa, suited for the introduction of a thermometer, syringe
needles, and optionally a mechanical stirrer) is charged with this
acrylic acid/crosslinker solution. Typically, a magnetic stirrer,
capable of mixing the whole content, is added. An amount of water
is calculated so that the total weight of all ingredients for the
polymerization equals 1500 g (i.e., the concentration of AA is 20
w/w-%). 300 mg of the initiator ("V50" from Waco Chemicals) are
dissolved in approx. 20 ml of this calculated amount of deionized
water. Most of the water is added to the resin kettle, and the
mixture is stirred until the monomer and water are well mixed.
Then, the initiator solution is added together with any remaining
water. The resin kettle is closed, and a pressure relief is
provided, e.g., by puncturing two syringe needles through the
septa. The solution is then spurged vigorously with argon via an 80
cm injection needle while stirring at .about.300 RPM. Stirring is
discontinued after .about.8 minutes, while argon spurging is
continued. The solution typically starts to gel after 12-20 minutes
total. At this point, persistent bubbles form on the surface of the
gel, and the argon injection needle is raised above the surface of
the gel. Purging with argon is continued at a lowered flow rate.
The temperature is monitored, typically it rises from 20.degree. C.
to 60-70.degree. C. within an hour. Once the temperature drops
below 60.degree. C., the kettle is transferred into a circulation
oven and kept at 60.degree. C. for 15-18 hours.
After this time, the resin kettle is allowed to cool, and the
resulting gel is removed into a flat glass dish. The gel is then
broken or cut with scissors into small pieces (for example in
pieces smaller than 2 mm max. dimension), and transferred into a 6
liter glass beaker. The amount of NaOH (50%) needed to neutralize
75% of the acid groups of the polymer is diluted with deionized
water to 2.5 liters, and added quickly to the gel. The gel is
stirred until all the liquid is absorbed; then, it is covered and
transferred into a 60.degree. C. oven and let equilibrate for 2
days.
After this time, the gel is allowed to cool, then divided up into 2
flat glass dishes, and transferred into a vacuum oven, where it is
dried at 100.degree. C./max. vacuum. Once the gel has reached a
constant weight (usually 3 days), it is ground using a mechanical
mill (e.g., IKA mill) and sieved to obtain hydrogel forming polymer
particles of the required particle size, e.g., 150-800 .mu.m. (At
this point, key parameters of the hydrogel forming polymer as used
herein may be determined).
The amount of MBAA may be adjusted, depending on what properties
are required from the resulting polymers, e.g., when 0.01 mol %
(per mol AA) MBAA is used, the resulting hydrogel forming polymer
particles have a CCRC of about 90 g/g (absorption of 0.9% saline
solution, as determined by methods known in the art and described
herein); when 0.03 mol % (per mol AA) MBAA is used, the resulting
hydrogel forming polymer particles have a CCRC of about 73 g/g;
when 0.1 mol % (per mol AA) MBAA is used, the resulting hydrogel
forming polymer particles have a CCRC of about 56 g/g; when 2.0 mol
% (per mol AA) MBAA is used, the resulting hydrogel forming polymer
particles have a CCRC of about 16 g/g; when 5.0 mol % (per mol AA)
MBAA is used, the resulting hydrogel forming polymer particles have
a CCRC of about 8 g/g. (All compounds were obtained by Aldrich
Chemicals, and used w/o purification.)
EXAMPLE 1.3
Surface Cross-Linking Process Step
This example demonstrates surface crosslinking of hydrogel forming
polymers prior to coating. A 150 ml glass beaker is equipped with a
mechanical stirrer with a plastic blade, and charged with 4 g of a
dry hydrogel forming polymer in particulate form. The mechanical
stirrer is selected in such a way that a good fluidization of the
polymers can be obtained at 300-500 RPM. A 50-200 .mu.l syringe is
charged with a 4% solution (w/w) of DENACOL
(=EthyleneGlycolDiGlycidylEther=EGDGE) in 1,2-propanediol; another
300 .mu.l syringe is charged with deionised water.
The hydrogel forming polymers are fluidized in the beaker at
approx. 300 RPM, and the surface cross-linking agent is added
within 30 seconds. Mixing is continued for a total of three
minutes. While stirring is continued, 300 .mu.l of water are then
added within 3-5 seconds, and stirring is continued at 300-500 RPM
for another 3 minutes. After this time, the mixture is transferred
into a glass vial, sealed with aluminum foil, and let equilibrate
for 1 hour. Then the vial is transferred to a 140.degree. C. oven,
and kept at this temperature for 120 minutes. After this time, the
vial is allowed to cool down, the contents is removed, and the
surface cross-linked hydrogel forming polymers are obtained. Any
agglomerates may be carefully broken by gentle mechanical action.
The resulting surface cross-linked hydrogel forming polymer
particles may then be sieved to the desired particle size.
The Following Examples Show Coating Processes that are Useful to
Make the Water-Swellable Material of the Invention
EXAMPLE 2.1
Process of Providing Water Swellable Materials with Coated Hydrogel
Forming Polymers by Directly Mixing Them into a Water Based Latex
Dispersion
The following is a preferred process for making the water-swellable
material of the invention, involving swelling the hydrogel forming
polymers prior to, or simultaneously with the coating step.
The amount of hydrogel forming polymers to be coated, coating level
and water needed to swell the hydrogel forming polymers is
chosen.
Then, the diluted dispersion of the coating agent is prepared,
e.g., of the latex as described herein. This is done by mixing the
commercial available elastomeric material and water and/or other
liquid (if required) under stirring, for example in a glass beaker
using magnetic stirrers at about 300 rpm for about 5 minutes. At
all times, care needs to be taken that no film is formed on the
surface of the dispersion. Typically for latex dispersions, the
dispersion contains at the most 70% by weight of wet-extensible
polymer.
In order to monitor the coating process better, a staining color
might be added to the dispersion, for example New Fuchsin Red.
Then, a mechanical stirrer with a double cross Teflon blade is used
and the dispersion is stirred such that a vortex can be seen, the
hydrogel forming polymer (particles) are quickly added under
continuous stirring. Once the hydrogel forming polymers start
absorbing the water from the dispersion (typically after about 15
seconds), the mixture will start to gel and the vortex will
eventually disappear. Then, when about all of the free liquid has
been absorbed, the stirring is stopped and the resulting coated
hydrogel forming polymers may be dried or post treated by any of
the methods described herein.
EXAMPLE 2.2
Process of Providing Individually Coated Water Swellable
Materials
An alternative preferred coating process of the invention is as
follows:
The (solid, particulate) hydrogel forming polymers are placed on a
surface that is preferably under an angle (30-45 degrees).
The coating agent, in the form of a dispersion, is applied in
drops, e.g., by use of a pipette or by spraying, onto the polymers.
Hereby, no air bubbles should be formed.
Thus, a film is formed on the surface of the hydrogel forming
polymers.
These coated hydrogel forming polymers are then dried, either at
room temperature (20.degree. C.), or for example at 40.degree.
C./80% humidity, for up to 2 days, or for example in an oven (if
required, a vacuum oven) at a low temperature (up to 80.degree.
C.).
The coated water swellable material can then be cured as described
below.
It may then also be formed into the desired form, e.g.,
particles.
EXAMPLE 2.3
Alternative Preferred Coating Process
In another preferred process, a dispersion of the hydrogel forming
polymers is prepared first and the coating agent is added
thereto.
For example, 200 grams of a hydrogel forming polymer (cross-linked
polyacrylic acid based polymers, for example prepared by the method
described above) is placed in a plastic beaker and n-heptane is
added, until the heptane stands about 1-2 mm above the surface of
the polymers in the beaker; this is typically about 100 g of
n-heptane.
Using a household mixer (e.g., for whipping cream), the components
are mixed at high speed. The coating agent, in the form of a water
dispersion of a wet-extensible coating material, e.g., a latex
dispersion as described above, is added to the beaker with the
hydrogel forming polymers by use of for example a pipette. The
mixture is continuously stirred, avoiding the formation of
lumps.
The resulting material can be spread out over a surface as a thin
layer (e.g., less than 1 cm) and allowed to air dry for at least 12
hours or in a (vacuum) oven (any temperature up to about 70.degree.
C.). The dried material may then additionally be cured by heating
to 140.degree. C. or 150.degree. C. in a (vacuum) oven.
After cooling or subsequent steps, the resulting material may be
mechanically reduced or sieved to the desired particle sizes.
EXAMPLE 2.4
Process of Providing Water-Swellable Material Comprising Coated
Hydrogel Forming Polymers, in Accord with the Invention, Using a
Fluidized Bed Wurster Coater
The coating process step may also be done in a fluidized bed or
Wurster coater.
For example, a GEA MP Micro coater (#99194) may be used (supplied
by Aeromatic-Fielder Ltd, School Lane, Chandlers Ford, Hampshire);
or a Glatt GPCG-3 granulator-coater may be used (supplied by Glatt
Ingenieurtechnik GmbH, Nordstrasse 12, 99427 Weimar, Germany). It
may be desired that the coating equipment is pre-heated, for
example to 70 C, under air flow, for example for about 30
minutes.
Then, typically between 20 and 35 gram of hydrogel forming polymer
is placed in the vessel (micro-coater case Makro; 2 kg).
The coating agent, preferably in fluid form (e.g., latex
dispersion) is placed in a container on the stirring platform and
stirred using a magnetic bar at low speed to prevent entrainment of
air. The weight can be recorded.
The peristaltic pump is calibrated and then set to the desired flow
rate (e.g., 0.1 g/minute) and the direction of flow of the coating
agent is set forward. The desired inlet air flow and temperature
are set (e.g., respectively 5 m.sup.3/h, to reduce the risk of
damage of the polymers; and, e.g., a temperature between 20 and
70.degree. C.). Then, the atomising air supply and pump are
started. (A higher speed may be used to advance the coating agent
closer towards the inlet of the coater and then setting the correct
speed for the experiment.)
The experiment is typically complete when stickiness prevents
efficient fluidisation of the powder (between 20 and 60
minutes).
Then, the coating agent flow is stopped immediately and flow
reversed. The weight of coating agent used in the experiment is
recorded.
Optionally, the resulting coated hydrogel forming polymers may be
dried within the coater, which also may aid to reduce particle
surface stickiness (drying time typically between 20 and 60
minutes).
Then, the material inside the coater is weighed.
In general, the material may be returned to the coating vessel to
continue the process, if required, e.g., if more than one coating
agent is to be applied or to add a flow aid, e.g., 0.5-2%
hydrophobic silica.
In order to visualise the coating process, or for aesthetic
purposes, a colouring agent or dye solution may be added to the
coating agent, for example New Fuchsin Red (0.25 g of New Fuchsin
Red in 5 ml of 25 ml deionised water (15-25.degree. C.), without
entrainment of air bubbles). The dye solution can be added
drop-wise to about 10 ml of the coating agent under stirring and
this can then be stirred into the remaining coating agent
(sufficient for up to 70 ml coating agent).
The following water-swellable materials were made by the process
above, using a fluid bed coater or Wurster coater; in each case, 25
g of the uncoated hydrogel forming polymers, available as GV-A640
from Nippon Shokubai (lot 0019H 0000 ISA0331) was used and the
specified amount of latex, at the specified weight-% solids
concentration, was used.
After drying of the coated samples for 2 days as 35.degree. C.,
each exemplified material was cured in vacuum at 140.degree. C. for
2 hours.
TABLE-US-00001 Latex concentration Amount of Example: Latex: (%
w/w): latex (g): 1 Hystretch V43 12.5% 5.0 2 Vinamul 330L 50% 2.5 3
Vinamul Elite 21 50% 2.5 4 Vinamul Elite 21 50% 5.5 5 Vinamul Elite
21 25% 3.0 6 Vinamul Elite 21 12.5% 4.5 7 Vinamul Elite 21 25% 3.0
8 Vinamul Elite 21 50% 3.5 9 75/25 PS:PB 20% 3.0 (experimental
latex) 10 Rovene 4151 12.5% 3.0 11 Rovene 4151 25% 2.0 12 GenFlo
3075 50% 2.5 13 GenFlo 3088 50% 1.0 14 Suncryl CP-75 50% 1.0
Hystretch is a trademark of Noveon Inc., 9911 Brecksville Road,
Cleveland, Ohio 44141-3247.
Vinamul is a trademark of Vinamul Polymers, De Asselen Kuil 20,
6161 RD Geleen, NL.
Rovene is a trademark of Mallard Creek Polymers, 14700 Mallard
Creek Road, Charlotte, N.C. 28262.
GenFlo and Suncryl are trademarks of Omnova Solutions Inc., 2990
Gilchrist Road, Akron, Ohio 44305-4418.
EXAMPLE 2.5
Preferred Subsequent Process Steps of Drying and Curing
The process of the invention may comprise a step whereby a
solution, suspension or dispersion or solution is used, e.g.,
whereby the (coated) hydrogel forming polymers comprise a liquid
(water) or whereby the coating agent is in the form of a
dispersion, suspension or solution.
The following is a preferred process step of drying the coated
hydrogel forming polymers of step b):
The coated hydrogel forming polymers or water-swellable material
comprising a liquid, e.g., water, is placed on a surface, for
example, it is spread out in a Pyrex glass pan in the form of a
layer which is not more than about 1 cm thick. This is dried at
about 70 Celsius for at least 12 hours.
If the amount of liquid present in the coated hydrogel forming
polymers/material is known, then, by measuring the coated
water-swellable material comprising said weight of liquid prior to
drying and then subsequently after drying, one can determine the
residual moisture in the resulting water-swellable material (coated
hydrogel forming polymers) as known in the art. Typically, the
resulting water-swellable material/coated hydrogel forming polymers
will be dried to less than 5% (by weight of the material) moisture
content.
The coated hydrogel forming polymers or material may subsequently
be cured, for example in a vacuum oven at 140 Celsius for 2
hours.
For some type of coating agents, coated hydrogel forming polymers
may potentially form agglomerates. Flow aids may be added prior to
or during the coating step, or preferably during the drying and/or
curing (annealing and/or cross-linking step), as known in the art,
e.g., Aerosil 200, available from Degussa.
The above drying step may also be done by spreading the coated
hydrogel forming polymers on a Teflon coated mesh in a very thin
layer, e.g., less than 5 mm, such as to enable convection through
the layer.
As alternative method, the coated hydrogel forming polymers that
contain a liquid (water), may also be directly dried and cured in
one step, e.g., placing the material in a vacuum oven at 140
Celsius for 2 hours.
EXAMPLE 2.6
Example: Method of Drying in Fluidized Bed
A Glatt coater as used in example 2.4 and other fluidized bed
driers known in the art may also be used to dry the coated
materials formed by step b) of the process. For example, the
conditions of example 2.4 might be used, introducing the coated
material (and thus using the Wurster coating equipment only for
drying the coated material).
EXAMPLE 2.7
Method of Azeotropic Distillation and Drying
The wet, coated polymers may be dried or dewatered at
low-temperature via azeotropic distillation from a suitable liquid,
for example cyclohexane. For example, the coated polymers are
transferred to a 2 liter resin kettle, equipped with a Trubore
mechanical stirrer with Teflon blade and digital stirring motor,
immersion thermometer, and Barrett type moisture receiver with
graduated sidearm and water-cooled condenser. Approximately one
liter of cyclohexane is added to the resin kettle. While stirring,
a heating mantle is used to raise the temperature of the stirred
cyclohexane/gel system to reflux. Reflux is continued until the
temperature of the system approaches the boiling point of
cyclohexane (approximately 80.degree. C.) and only minimal
additional quantity of water is delivered to the sidearm. The
system is cooled and then filtered to obtain the dewatered or dried
coated hydrogel forming polymers, which may be further dried
overnight under vacuum at ambient temperature (20 C).
Test Methods Used Herein:
(Unless specified otherwise, each test to obtain a value parameter
herein is done 3 times to obtain an average of 3 values.)
Methods to Determine Whether an Elastomeric Coating is
Non-Breaking
The following method is used to determine whether the elastomeric
coating of hydrogel forming polymers comprised in the
water-swellable material of the invention is non breaking. This is
done by first staining and then swelling the water-swellable
material and thus the coated hydrogel forming polymers therein, and
then investigating the elastomeric coating on the swollen hydrogel
forming polymers, by the following method.
A) Staining of the water-swellable material of the invention with
Toluidine Blue in 0.9% NaCl
Staining Solution (20 PPM Toluidine Blue O in 0.9% NaCl):
20 mg Toluidine Blue O [CAS: 540-23-8] is dissolved in 250 ml 0.9%
(w/w) NaCl solution. The mixture is placed into an ultrasonic bath
for 1 hour, filtered through a paper filter, and filled up to 1000
ml with 0.9% NaCl solution.
Staining Procedure:
A sample of 30-50 mg of the water-swellable material is placed in
screw cap glass vial, and 30 ml of the above staining solution is
added. The vial is closed, and the material is allowed to swell and
equilibrate for 18 hours at 20-25.degree. C. during gentle
agitation (e.g., gentle swirling or slow rolling of the vial on a
roller mill).
B) Alternative Oxidative Staining Method with
MnO.sub.4.sup.--Solution:
An alternative visualization of the elastomer coating material can,
e.g., be achieved by an oxidative staining with
MnO.sub.4.sup.-.
Hereby, the water-swellable material of the invention is first
swollen in a 0.9% NaCl solution:
A sample of 30-50 mg of the water-swellable material of the
invention is placed into a 40 ml screw cap glass vial, and 30 ml of
0.9% NaCl solution in water are added. The vial closed and the
material is allowed to swell and equilibrate at 20-25.degree. C.
for 18 hours under gentle agitation via occasional gentle shaking
or rolling on a roller mill.
Oxidative Staining Procedure:
Then, the swollen material is stained as follows: 20-25 mg
KMnO.sub.4 are dissolved in 100 ml 0.9% (w/w) NaCl solution.
Excess liquid is removed from the swollen water-swellable material
(e.g., by letting it drip off) and 30 ml of the KMnO.sub.4 solution
is added, while gently swirling of the solution for a few minutes.
This gentle swirling is repeated at intervals of 15-20 minutes.
After 60-90 minutes, the staining solution is removed, and the AGM
is washed several times with 0.9% NaCl solution. As some
MnO.sub.4.sup.- will bleed from the swollen particles, washings may
have to be repeated in intervals of 10-15 minutes. The staining is
complete when the supernatant does not pick up a noticeable purple
color any more.
C) Alternative Oxidative Staining Method Using OsO.sub.4 (or
RuO.sub.4)
The water-swellable material is swollen as in B above.
Then, a few drops of a solution of OsO.sub.4 in water (4% w/w) are
then added to the swollen material, and gently swirled for 30 min-1
day. Then, the staining solution is removed, and replaced by 0.9%
saline. After 1 hour, the solution is discarded, and the saline
solution is added one more time to remove excess oxidant.
Similarly, RuO.sub.4, freshly prepared from RuO.sub.2 or RuCl.sub.3
(following procedures described in for example "Polymer
Microscopy", Linda C. Sawyer, David T. Grubb, Chapman and Hall, New
York, ISBN 0 412 25710 6), may be used for staining, especially for
elastomeric coatings rich in aromatic moieties (e.g., styrene-rich
elastomeric coatings).
Assessment of the Percentage Non-Breaking Coating
For microscopy assessment, a swollen, stained sample as prepared by
any of the methods above is weighed and then transferred into white
porcelain dishes, and covered with the solution in which they were
prepared (or placed into 1 cm glass or quartz cuvettes with a
stopper in contact with this solution).
Separately, un-coated hydrogel forming polymers may also be
submitted to the respective staining method and this assessment for
comparison.
A stereomicroscope (e.g., Olympus Stereomicroscope SZH10
(7-70.times.), equipped with a circular illumination (e.g.,
Intralux UX 6000-1, Volpi AG, CH 8952 Schlieren, Switzerland), and
optionally a camera (e.g., Olympus ColorView 12), may be used for
evaluation of the swollen, stained, coated AGM particles.
As described above, with this equipment the particles with a
non-breaking coating can be distinguished, visually, from the
material without non-breaking coating. Then, the particles with a
non-breaking coating are separated from those without non-breaking
coating and the two fractions are weighed; the weight of the two
fractions together is the total weight of the sample; then, the
weight percentage (compared to the total weight) of the particles
with the non-breaking coating can be calculated.
To obtain a meaningful percentage of particles with a non-breaking
coating, the total of solids (e.g., particles) observed should be
at least 100.
Determination of Cylinder Centrifuge Retention Capacity of
Water-Swellable Materials
This test serves to measure the saline-water-solution retention
capacity of the water-swellable material or hydrogel forming
polymers used herein, when the water-containing material (polymer)
are submitted to centrifuge forces (and it is an indication of the
maintenance of the absorption capacity of the polymers in use, when
also various forces are applied to the material).
First, a saline-water solution is prepared as follows: 18.00 g of
sodium chloride is weighed and added into a two liter volumetric
flask, which is then filled to volume with 2 liter deionised water
under stirring until all sodium chloride is dissolved.
A pan with a minimum 5 cm depth, and large enough to hold four
centrifuge cylinders is filled with part of the saline solution,
such that up to a level of 40 mm (.+-.3 mm).
Each sample is tested in a separate cylinder and each cylinder to
be used is thus weighed before any sample is placed in it, with an
accuracy of 0.01 g. The cylinders have a very fine mesh on the
bottom, to allow fluid to leave the cylinder.
For each measurement, a duplicate test is done at the same time; so
two samples are always prepared as follows:
1.0 g of the water-swellable material (or hydrogel forming
polymers) which is to be tested is weighed, with an accuracy of
0.005 g (this is the `sample`), and then the sample is transferred
to an empty, weighed cylinder. (This is repeated for the
replica.)
Directly after transferring the sample to a cylinder, the filled
cylinder is placed into the pan with the saline solution (Cylinders
should not be placed against each other or against the wall of the
pan.).
After 15 min (.+-.30 s), the cylinder is removed from the pan, and
the saline solution is allowed to drain off the cylinder, then, the
cylinder is re-placed in the pan for another 15 min. After the
total of 2.times.15 minutes=30 minutes immersion time, the cylinder
is taken from the solution and excess water is allowed to run off
the cylinder and then, the cylinder with the sample is placed in
the cylinder stands inside a centrifuge, such that the two
replicate samples are in opposite positions.
The centrifuge used may be any centrifuge equipped to fit the
cylinder and cylinder stand into a centrifuge cup that catches the
emerging liquid from the cylinder and capable of delivering a
centrifugal acceleration of 250 G (.+-.5 G) applied to a mass
placed on the bottom of the cylinder stand (e.g., 1300 rpm for a
internal diameter of 264 mm). A suitable centrifuge is Heraeus
Megafuge 1.0 VWR #5211560. The centrifuge is set to obtain a 250 G
centrifugal acceleration. For a Heraeus Megafuge 1.0, with a rotor
diameter of 264 mm, the setting of the centrifuge is 1300 rpm.
The samples are centrifuged for 3 minutes at 250 G (.+-.10 s).
The cylinders are removed from the centrifuge and weighed to the
nearest 0.01 g.
For each sample (i), the cylinder centrifuge retention capacity Wi,
expressed as grams of saline-water-solution absorbed per gram of
water-swellable material (or hydrogel forming polymer) is
calculated as follows:
.function. ##EQU00001## where: m.sub.CS: is the mass of the
cylinder with sample after centrifugation [g] m.sub.Cb: is the mass
of the dry cylinder without sample [g] m.sub.S: is the mass of the
sample without saline solution [g]
Then, the average of the two W.sub.i values for the sample and its
replica is calculated (to the nearest 0.01 g/g) and this is the
CCRC as referred to herein.
Method to Determine the Free Swell Rate of Water Swellable
Materials Herein
This method serves to determine the swell rate of the
water-swellable materials herein in a 0.9% saline solution, without
stirring or confining pressure. The amount of time taken to absorb
a certain amount of fluid is recorded and this is reported in gram
of fluid (0.9% saline) absorbed per gram of water-swellable
material per second, e.g., g/g/sec.
The saline solution is prepared by adding 9.0 gram of NaCl into
1000 ml distilled, deionized water, and this is stirred until all
NaCl is dissolved.
1.0 gram of the sample material is weighed (to an accuracy of 0.001
g) and placed evenly over the bottom of a 25 ml beaker; then 20 ml
of the saline solution (also at 23 C) is added quickly to the
beaker with the sample and the timer is started.
When the last part of the undisturbed fluid surface meets the
swelling sample, e.g. judged by the light reflection from the fluid
surface, the time t.sub.s is recorded.
The test is repeated twice, to obtain 3 values.
The Free Swell Rate is then calculated per sample and this can be
averaged to obtain the Free swell rate, as referred herein.
Determination of the Coating Caliper and Coating Caliper
Uniformity
The coatings on the hydrogel forming polymers of the
water-swellable materials herein can typically be investigated by
standard scanning electron microscopy, preferably environmental
scanning electron microscopy (ESEM) as known to those skilled in
the art. In the following method the ESEM evaluation may be used to
determine the average coating caliper and the coating caliper
uniformity of the coated hydrogel forming polymers via
cross-section of the materials.
Equipment Model: ESEM XL 30 FEG (Field Emission Gun)
ESEM setting: high vacuum mode with gold covered samples to obtain
also images at low magnification (35.times.) and ESEM dry mode with
LFD (large Field Detector which detects .about.80% Gaseous
Secondary Electrons+20% Secondary Electrons) and bullet without PLA
(Pressure Limiting Aperture) to obtain images of the latex shells
as they are (no gold coverage required).
Filament Tension: 3 KV in high vacuum mode and 12 KV in ESEM dry
mode.
Pressure in Chamber on the ESEM dry mode: from 0.3 Torr to 1 Torr
on gelatinous samples and from 0.8 to 1 Torr for other samples.
Samples of coated water-swellable material or hydrogel forming
polymers or of uncoated polymers can be observed after about 1 hour
at ambient conditions (20 C, 80% relative humidity) using the
standard ESEM conditions/equipment.
Then, the same samples can be observed in high vacuum mode.
Then the samples of coated hydrogel forming polymers can be cut via
a cross-sectional cut with a Teflon blade (Teflon blades are
available from the AGAR scientific catalogue (ASSING) with
reference code T5332), and observed again under vacuum mode.
The coatings have different morphology than the uncoated hydrogel
forming polymers and the coatings are clearly visible in the ESEM
images, in particular when observing the cross-sectional views.
The average coating caliper is determined then by analyzing at
least 5 particles of the hydrogel forming polymer, coated with a
non-breaking coating, and determining 5 average calipers, an
average per particle (by analyzing the cross-section of each
particle and measuring the caliper of the coating in at least 3
different areas) and taking then the average of these 5 average
calipers.
The uniformity of the coating is determined by determining the
minimum and maximum caliper of the coating via ESEM of the
cross-sectional cuts of at least 5 different particles of hydrogel
forming polymers, coated with a non-breaking coating, and
determining the average (over 5) minimum and average maximum
caliper and the ratio thereof.
If the coating is not clearly visible in ESEM, then other staining
techniques known to the skilled in the art that are specific for
the coating applied may be used such as enhancing the contrast with
osmiumtetraoxide, potassium permanganate and the like, e.g. prior
to using the ESEM method.
The dimensions and values disclosed herein are not to be understood
as being strictly limited to the exact numerical values recited.
Instead, unless otherwise specified, each such dimension is
intended to mean both the recited value and a functionally
equivalent range surrounding that value. For example, a dimension
disclosed as "40 mm" is intended to mean "about 40 mm."
All documents cited in the Detailed Description of the Invention
are, in relevant part, incorporated herein by reference; the
citation of any document is not to be construed as an admission
that it is prior art with respect to the present invention. To the
extent that any meaning or definition of a term in this document
conflicts with any meaning or definition of the same term in a
document incorporated by reference, the meaning or definition
assigned to that term in this document shall govern.
While particular embodiments of the present invention have been
illustrated and described, it would be obvious to those skilled in
the art that various other changes and modifications can be made
without departing from the spirit and scope of the invention. It is
therefore intended to cover in the appended claims all such changes
and modifications that are within the scope of this invention.
* * * * *